IP-based mobile networks are characterized by a scenario in which a mobile user moves within the network and generates traffic that is routed inside the network towards the nodes with which he is communicating (correspondent nodes). During his movements, the user can connect with networks different from that of his service provider or operator. In that case there is also the possibility for the user of entering an unknown access network with which no agreement exists in terms of movement management or roaming.
In such a situation the need arises of making the user reachable at any instant and anywhere he may be, and of keeping active all the communication sessions that the user has in progress.
During his movements, the user may have to change the access network (sub-network) that enables him to use IP services. This operation must be transparent to the user so that he can continue communicating with the correspondent nodes without interruptions.
Due to their intrinsic nature, the conventional protocols used as the basis of IP networks were not capable of managing IP terminals moving within the network. To fill up this gap, the standardization group known as IETF (Internet for Engineering Task Force) has worked out the Mobile IP protocol, that allows a mobile terminal to change its point of attachment to the network in a transparent manner with respect to the applications. The Mobile IP protocol is available for both the IPv4-based networks as Mobile IPv4, and for IPv6-based networks as Mobile IPv6.
Mobile IPv6 is specified in the document draft-ietf-mobileip-ipv6-24. This is the first reference made in the present description to draft . . . or RFC . . . standards: any relating information will be laid open at the filing date of this application at the IETF web site under the address http://www.ietf.org.
In case Mobile IPv6 is adopted, two IP addresses must be assigned to the mobile node: the first address is its Home Address (HoA), which never changes and is used to uniquely identify the identity of the node (in the sequel also referred as mobile node or terminal), while the second address is the Care-of Address (CoA), an address belonging to the visited subnet used to identify the actual position of the mobile terminal.
Any movement implying a change in the IP subnet being visited requires that the mobile terminal registers its own Care-of Address at a server, called Home Agent (HA), present on its provider's network (also called “home network”). Any other IP terminal trying to communicate with the mobile node by contacting it on the provider's network of the same mobile node (through the Home Address) is re-directed by the Home Agent HA towards the actual position of the mobile node, which is identified by the Care-of Address. In this way all the traffic directed to the mobile node is sent by the Home Agent to the current address of the user, i.e. the Care-of Address. Thus, the mobile node is constantly reachable, regardless of its connection point to the network.
FIG. 1 shows a generic scenario of Mobile IPv6 usage within the IP network hosting the mobile node.
In particular, in FIG. 1 the mobile node MN (Mobile Node) denoted by 100 is linked to the access network denoted by 104.
Network 104 allows mobile node 100 to establish a connection with the network 112 of its provider and in more detail it allows it to establish a communication session 102 with a particular server, called Home Agent (HA) and denoted by 114 in FIG. 1.
References 106, 108 and 110 indicate other possible networks (subnetworks) for accessing the IP network. Furthermore, reference 116 denotes an interconnection network between the network 112 of the provider of mobile node 100 and the network 118 of another provider. The movement of mobile node 100 within the network is indicated by the arrow 120.
Mobile IPv6 defines two communication modes between mobile node 100 and the nodes, called “correspondent nodes”, with which node 100 is communicating. These two modes are illustrated in FIG. 2.
The first mode, referred to as Bi-directional Tunneling (BT), requires the movements of mobile node 100 to be known only to its Home Agent 114; therefore all the traffic relating to mobile node 100 is intercepted by Home Agent 114, which re-directs it to the correct destination, i.e. towards the Care-of Address of the mobile node.
In this way, all the traffic generated and received by mobile node 100 goes through the provider's network 112 of the same mobile node. In FIG. 2, the traffic path followed in this mode, called Bi-directional Tunneling, is depicted by a broken line and denoted by reference 124.
The second mode, referred to as Route Optimization (RO), implies instead that by any of its movements the mobile node 100 shall send information about its new position, i.e. the new Care-of Address, also to the correspondent nodes 130. In this way said nodes 130 can exchange traffic directly with mobile node 100, as they know its address within the network being visited (i.e. the Care-of Address).
Consequently, the traffic directed to mobile node 100 has no longer to go necessarily through the provider's network 112, but may follow the “optimized” path indicated by means of a continuous line in FIG. 2 and denoted by reference 122. All this is to take place by an at least partial exclusion of the provider's network 112.
The possibility of communicating through the second mode, i.e. Route Optimization, has been introduced into Mobile IPv6 because it allows the optimization of traffic routing. This normally results in an improvement in the performance experienced by the users, owing to a better management of the network resources, which prevents the overload of the provider's network relating to the mobile node by virtue of an at least partial exclusion of provider's network 112 from the routing path.
According to the Mobile IPv6 specifications, the choice of communicating in Bi-directional Tunneling mode or in Route Optimization mode is completely left to the mobile terminal, and the provider's network (i.e. the network denoted by 112 in FIGS. 1 and 2) does not have any means of intervention on such a choice.
This operational behavior perfectly fits into the case wherein the user moves within the network of his provider (e.g. a mobile operator or an Internet Service Provider (ISP)) or inside the networks of other providers with which roaming agreements are in place. In fact, in all these cases, regardless of the routing mode (i.e. Bi-directional Tunneling or Route Optimization), the provider with which the mobile user has subscribed to the service, with a possible co-operation of the provider managing the visited network, is always in a position to monitor and check the resulting traffic. All the above is devised to guarantee, whenever the connection point to the network varies, the uniformity of the service supplied and the possibility of charging accordingly.
However, in a wider mobility scenario, where movements are also possible towards local networks (e.g., company or corporate networks, free access public networks) with which no roaming agreements are in place, it may happen that in case of communication in the Route Optimization the service provider is no longer aware of the typology and quantity of the traffic generated by the user. This applies even if the exclusion of the supplier under question is not complete, but only partial: as a matter of fact, the supplier keeps allocating resources in order to ensure the user a set of value-added services, such as the guarantee of reachability at his own Home Address, and mobility management during movements.
Missing sufficient information for ensuring the uniformity of the service supplied, the provider may choose to interrupt the service at the Home Agent when the user is connecting to an access network with which no roaming agreements are in place (i.e. an unknown access network).
Obviously this situation is disadvantageous for the user who may not be actually provided with the mobility level on which he relies, since the service continuity can no longer be guaranteed.